The pleura is a thin, transparent, two-layered membrane that covers the lungs and also lines the inside of the chest wall. The layer that covers the lungs lies in close contact with the layer that lines the chest wall. Between the two thin flexible layers is a small amount of fluid that lubricates them as they slide smoothly over one another with each breath.
In abnormal circumstances, air or excess fluid can get between the pleural surfaces, creating a space. If excess fluid accumulates (called pleural effusion) or if air accumulates (called pneumothorax), one or both lungs may not be able to expand normally with breathing, resulting in the collapse of lung tissue.
The mediastinum refers to an area that is bordered by the breastbone (sternum) in front, the spinal column in back, the neck on top, and the diaphragm below. It contains the heart, the thymus gland, some lymph nodes, and parts of the windpipe (trachea), esophagus, aorta, thyroid gland, and parathyroid glands. It does not include the lungs. The mediastinum is divided into three parts: front (anterior), middle, and back (posterior).
Various abnormal masses such as cysts and tumors can form in the mediastinum. The mediastinum may also be irritated if contents from the esophagus leak into it.
Last full review/revision November 2012 by Richard W. Light, MD